In the printed circuit board (PCB) industry, it is known to use laser plotters for exposing a photographic film so as to produce an artwork relating to a specific layer of a PCB to be manufactured. The PCB may comprise several layers each of which must be aligned precisely with respect to one another. Since each layer relates to a specific artwork, the photographic films which correspond to each of the layers must themselves be aligned in precise registration.
Such registration is achieved by punching holes in the photographic film in precise registration with the holes of other films relating to other layers of the same PCB. The exposed film bearing the artwork for each layer is registered on a surface having corresponding registration pins. Since each of the artworks is aligned with the surface in this manner, they are also aligned with respect to each other, thereby ensuring that all of the layers of the resulting PCB are in correct alignment.
Furthermore, since PCBs are mass-manufactured by employing several artworks simultaneously, it is important that the registration holes produced in the artworks are precisely aligned not only with respect to the corresponding layers of the PCB, but also with respect to each other. In other words, not only is precision required but so also is repeatability.
Several prior art methods and systems exist for punching the registration holes in the film. Thus, it is known to punch the registration holes before the film is inserted into the plotter. This, of course, is analogous to the sprocket holes produced in camera films, permitting such films to be wound on in the camera. The main drawback with such an approach is that, since the film is punched prior to exposure, the punching must be performed in a dark room.
Alternatively, the holes may be punched after the film is exposed. However, this requires the artwork to be aligned with the punching machine. Since the very purpose of the holes is to permit precise registration of successive artworks during subsequent manufacture, it is clearly unsatisfactory to employ a method which requires that the exposed film be aligned with the punching machine first in order for the registration holes to be punched. It is known to achieve such alignment automatically using electronic vision systems, but the operation is relatively slow and costly.
In order to overcome this problem, it is known, in flat bed plotters, to punch the holes in precise registration with the artwork whilst the film is on the plotter bed. There are several advantages to such an approach. First, it obviates the need for a dedicated punching machine. Secondly, the alignment is achieved automatically without the need for a costly and complicated aligning system and, thirdly, it saves time in the production process since the punching and the plotting are produced in a single stage of the manufacturing process whilst the provision of a separate punching machine clearly imposes an additional stage in the manufacturing process.
In such flat bed plotters, a conventional punch and die is provided in respect of each hole to be punched and each punch and die is pre-aligned in precise registration. The dies are formed within the surface of the plotter bed towards an edge thereof, and the punch mechanism is fixedly attached to the end of the plotter bed. In such an arrangement when the plotter bed moves so too does the punch mechanism so as to maintain the precise registration between the punch and dies.
FIGS. 1 and 2 show a conventional system for punching holes through a film 1 using a die 2 and punch 3. In this system, the punch 3 is located on one side of the film 1 and the die 2 on the other, and a hole 4 is produced by inserting the punch 3 through the film 1 into the die 2. Such a system is suitable only if the clearance between the punch 3 and the die 2 is small enough, i.e. less than 10% of the film thickness.
Thus, denoting:
t=the thickness of the film, PA1 Dp=the diameter of the punch, and PA1 Dd=the diameter of the die PA1 (a) providing for each hole to be punched a corresponding socket in said working surface, PA1 (b) securing the sheet material on the working surface so as completely to overlap all of the sockets, PA1 (c) forming the artwork on the sheet material in precise registration with a predetermined origin of the working surface, PA1 (d) associating with each of the sockets a punch and block combination disposed on opposite surfaces of the sheet material, said block being formed of a softer material than the punch and having an operating surface larger than a cutting surface of the punch, PA1 (e) for each of said punch and block combinations, aligning the punch with the block so that the operating surface of the block completely overlaps the cutting surface of the punch, and PA1 (f) effecting relative axial displacement of the punch towards the block aligned therewith so as to punch a hole through the sheet material in precise registration with the artwork.
it is required that: EQU Dd-Dp.ltoreq.0.1t
When punching the registration holes in accordance with such a system, the punch, together with its actuator, is attached to the frame of the plotter and the die is located within the working surface thereof. The main disadvantage of such an approach is the very high accuracy required for aligning the punch with the die. Thus, for example, a film thickness of 100 .mu.m requires that the tolerance of the punch location in relation to the die be at least 5 .mu.m.
Such an arrangement is rendered feasible only because the punch mechanism is fixed to the plotter bed since, were this not the case, it would not be possible to maintain the precise registration between the punch and dies which is essential. The punch mechanism is generally robust and heavy and thus must also be effectively supported by the plotter's transport mechanism which moves the plotter bed through mutually orthogonal X and Y axes in order to align a desired co-ordinate on the film with a fixed writing head. As is known, such flat bed plotters move relatively slowly in the X and Y directions and therefore the extra mass of the punch mechanism and its consequent load on the plotter's transport mechanism can easily be tolerated.
However, this is by no means the case where drum plotters are concerned for two reasons. First, drum plotters rotate at very high speed and therefore any extra loading on the drum's transport mechanism is unacceptable. Secondly, fixing a heavy punch mechanism to a periphery of the drum would unbalance the drum and unacceptably affect the drum's dynamics.
Regardless of whether a flat bed or drum plotter is used to plot the artwork, a slight misalignment of the punch with the die not only causes inaccuracies in the resulting registration holes but, on account of the force with which the punch is inserted into the die, can damage the punch itself of even the working surface of the plotter which is made of a softer material than the punch.
U.S. Pat. NO. 4,485,713 (Dotta) discloses a punching device comprising a pair of opposed complementary dies vertically guided by a pair of guide rods each rotatably connected to a crank pin to continuously punch band material moving between the dies.
In use, a band material of paper, for example, is drawn by a pair of rollers through the gap between the opposing dies and punches whilst, at the same time, a pair of toothed wheels rotate so that the punches connected eccentrically thereto perform linear vertical to-and-fro motion along a pair of guide rods thereby successively engaging and disengaging with the dies. The dies and punches must be in exact registration with one another, this being a prerequisite of any punching machine.
By such means, holes are punched into the band material at regularly spaced intervals, depending on the traverse speed with which the band material is drawn through the device.
Furthermore, in the Dotta system, each punch finds the center of a corresponding die in much the same way that a drill bit locates a pre-formed hole or center punch. This is achieved by mounting the punches in a fluid bed so as to provide some lateral play. Thus, providing that the tip of each punch finds the center of the corresponding die, the punch can move laterally slightly in order to ensure exact alignment with the die. It is thus apparent that one of Dotta's objectives is to ensure precise alignment between the punch and die regardless of any initial fractional misalignment.
Furthermore, in the Dotta arrangement the respective dies and punches are mounted on completely planar surfaces and are axially brought into engagement and disengagement along the pair of parallel guide rods firmly connected perpendicular to the ends of one of the dies.
It will readily be apparent that the Dotta system is massive and certainly not adaptable for use with a drum plotter whose drum rotates at high speed and requires precise balancing.
German Patent No. DT 25 49 937 discloses a punch mounted on a cylindrical surface and which impacts a flexible substrate which is larger than the punch.
DT 25 49 937 obviates the need for precise alignment between the punch and die. However, there is nothing in this reference to suggest its application to a system for plotting an artwork and for punching registration holes therethrough in precise registration with the artwork. There is further no reason to combine the teachings of DT 25 49 937 with those of the Dotta system, since DT 25 49 937 actually teaches away from the teachings of Dotta in that in the Dotta system precise alignment between the punches and dies is required, whilst DT 25 49 937 obviates the need for such precise alignment.
Even apart from these considerations, neither of these references is suitable, either singly or in combination, for ensuring that the punches holes are in precise registration with an artwork previously or subsequently formed on a film material disposed on a plotter. They are merely directed to ensuring registration, or alternatively to obviating the need for such registration, between the punch and die.
U.S. Pat. No. 3,618,438 (Simson) is directed to cutting webs of sheet-like material in a web offset printing press simultaneously with the action of applying ink images to one or both sides of the web of sheet-like material.
Thus, as shown in Simson, a web of sheet-like material is fed between two interacting rollers referred to, respectively, as a blanket cylinder and an impression cylinder. A die having a protruding cutting edge is secured to the impression cylinder so that on encountering the sheet material against an opposing surface of the blanket cylinder, a section is cut out of the sheet material corresponding to the contour of the die.
Here again, there is no suggestion that the removed section of the sheet material is in a predetermined and precise registration with the sheet material itself still less, of course, with an artwork formed thereon either prior or subsequent to punching the holes.
It will be borne in mind that in plotters used in the PCB manufacturing industry, the print head is mounted in precise and known registration with respect to the working surface. Consequently, if the punches in particular are mounted within the working surface, their location must be in known and precise registration relative to the printing head. This guarantees that the holes thus punched through the artwork are in precise registration therewith since, obviously, the artwork is registered relative to a rest position of the print head. This notwithstanding, no solution has yet been proposed to providing a punch and die combination integral with the working surface of a plotter which exploits this fact and which is, at the same time, both light and amenable to application in drum plotters which have the very rigid dynamic requirements as outlined above.